A harmonic-balance-based large-signal model of the heterojunction bipolar transistor (HBT) suitable for microwave circuit design and simulation has been developed. Equivalent circuits were extracted from measured small-signal S-parameters by equivalent circuit fitting and used in a harmonic-balance-based simulator. Modeled power and gain results were in good agreement with directly measured characteristics. The power saturation mechanisms have been established in terms of current/voltage amplitudes and power-dependent equivalent circuit elements. Saturation has been shown to be due to the signal-voltage swing entering the cutoff or saturation regions with the resulting loss of current gain. It is pointed out that the model can accurately predict experimentally measured AlGaAs/GaAs HBT power characteristics.